The present invention relates to a technique of conveying an object to be conveyed by a motor.
Conventionally, there are known conveyance devices such as an inkjet printer that conveys a recording sheet or a recording head by a driving force of a motor thereby to form an image on the recording sheet, and a scanner that conveys a line sensor in a sub-scanning direction by a motor thereby to read a document.
In an inkjet printer, for example, there is a limitation in an amount of image that a recording head can form at a time. Therefore, a recording sheet is intermittently conveyed to a recording position and an image is formed in a stepwise manner on the recording sheet, so that the image is formed on the whole recording sheet.
Upon such stepwise recording operation, however, continuity of image cannot be maintained if the recording sheet is not conveyed with high precision. As a result, image quality may be deteriorated. Especially in recent years, a high-resolution image can be recorded on a recording sheet due to advancement in micronization of ink drops. Accordingly, there is also a demand for high precision in a feeding amount of a recording sheet.
In recent-years, a recording sheet is fed by a fixed amount with precision by rotating a motor at a very low speed in a vicinity of a stop position of the recording sheet. At the same time, a conveyance speed of the recording sheet is measured so that a manipulated variable (driving voltage or current) to the motor is determined based on a measurement result.
Particularly, once a leading edge of an encoder signal outputted from an encoder attached to the motor is detected, elapsed time from a previously detected leading edge is specified. The conveyance speed is specified from the elapsed time. The driving voltage or current corresponding to a difference between the conveyance speed and a target speed is then inputted to the motor thereby to drive the motor.
However, in case that the motor is driven in the above-described manner, the leading edge of the encoder signal is not detected when a load is applied to the motor, and an object to be conveyed is abruptly stopped. Then, speed information of the object to be conveyed is not updated. As a result, the difference between the conveyance speed and the target speed is not increased although the object to be conveyed is stopped. The motor is continued to be driven by a low driving voltage or current which is insufficient to move the object to be conveyed. Conveyance of the object to be conveyed may not be able to be restarted.
Such problem occurs not only in an inkjet printer but also in a scanner. Specifically, in the field of scanner, there are more and more demands for high scanning resolution these days. Accordingly, there is a need to convey a line sensor a an extremely low speed. In case that the line sensor is conveyed at an extremely low speed, however, the object to be conveyed (and the motor) is abruptly stopped by a slight load change since a torque of the motor is small. The same problem as in an inkjet printer occurs.
In order to handle such problem, in one prior art, in case that the speed information is not updated within a prescribed period of time, it is determined that a time out has occurred, so as to increase the driving voltage inputted to the motor.